The present invention relates to a rear projection screen for use in a projection television set and the like.
As shown in FIG. 15, a rear projector used in a video projection television set is adapted to expand light emerging from a light source P, e.g. a CRT, by means of a lens system L, project the light from the rear side of a screen S and allow a viewer to observe a picture from the opposite side of the screen S. However, if the distance from the light source P to the screen S is prolonged as shown, the projector becomes large size. In practice, therefore, a method is adapted whereby one to three mirrors M are combined, and the light is projected after being reflected on them once, as shown in FIGS. 16A, 16B, 16C. However, there have been drawbacks in that, in the method of FIG. 16A, the height of the projector becomes large, while, in the methods of FIGS. 16B and 16C, it cannot be said positively that the projectors have been made sufficiently compact in terms of the height and depth.
To overcome these drawbacks, a rear projection screen has been proposed for observing an image by introducing light at a sharp angle from the rear, characterized in that a plurality of prisms are provided in parallel with each other on the light incidence surface, each having a total reflection surface, so that the incident light is totally reflected on the total reflection surface of the screen and is then emerged from the front viewing surface of the screen (refer to Japanese Pat. Application No. 29964/1984, U.S. Pat. Application Nos. 837,412 filed March 7, 1986, now U.S. Pat. No. 4,674,836, issued June 23, 1987 and 925,723).
Explanation will be made to the abovementioned rear projection screen proposed by the abovementioned U.S. Pat. Application Nos. 837,412 and 925,723 with reference to FIG. 1 in which like reference characters designate like or corresponding parts as shown in FIG. 15.
Light introduced from a light source P is made incident upon the rear surface of the rear projection screen S at a relatively large incident angle. The angle .theta. of incidence is generally dependent upon the distance of light projection, but preferably is in a range from 40to 85 deg. The distance 1' from the light source P to the rear projection screen S can be made to be shorter than the distance 1 shown in FIG. 15 by a relatively large incident angle .theta. within a range as mentioned above or by making the light from the light source P incident upon the rear surface of the screen S at an acute angle to the latter. Since the light source P is located obliquely downward, a distance 1' in the direction of the depth is: EQU 1'=1 cos .theta.
so that e', can be made relatively small in comparison with e.
However, since the height of the projection set may not be always made sufficiently low, it is desirable in practice to reduce the height by employing a mirror M.sub.1, as shown in FIG. 2A, and to reduce the length in the direction of the depth as well. In addition, in order to further reduce the height so as to make the overall dimensions smaller, two mirrors M.sub.2 and M.sub.3 may be combined as shown in FIG. 2B, and the light source P may be interposed between the rear projection screen S and the first mirror M.sub.2 so as to project the light after being reflected twice.
FIG. 3 illustrates a portion of the rear projection screen mentioned above, and, a plurality of prisms having the same configuration are provided on the rear surface of the rear projection screen. In other words, these prisms are constituted by a plurality of prisms 1 extending along lines or concentric circular arcs in parallel and each prism 1 having a light incidence surface 1B and a light reflection surface 1A. A total reflection surface is formed on the light reflection surface 1A in such a manner that the light entering from the light incidence surface 1B is totally reflected and is then emerged from a front viewing surface.
As can be seen in FIG. 1, the light ray incident upon the rear projection screen S at the lower edge thereof has an incidence angle smaller than that of the light ray incident at the upper edge thereof. That is, the higher the incident position of the light ray on the rear projection screen, the greater the incidence angle of the right ray becomes, in other word, the higher the incident position of the light ray, the lower the density of the light rays incident upon the rear projection screen would becomes, resulting in that a picture image projected on the screen from the light source cannot be uniformly focused over the entire surface of the rear projection screen if no correction measure is taken for the light source P. However, optical measures or electric measures are proposed to overcome the above-mentioned problem.
However, there is a drawback in the above-mentioned rear projection screen. That is, if the incidence angle of the light ray is comparatively small or if the relative height of the prisms is small with respect to the pitches thereof, that is the apex angle .theta..sub.1 of the prisms is large, a part of light rays incident upon the rear surface of the screen S directly enters into the medium of the screen S without reflecting upon the light reflection surfaces 1A of the prisms. More specifically, referring to FIG. 4, of the light rays .alpha., the part of light rays .beta. reflects upon the light reflection surface 1A of each prism and emerges from the screen as effective light rays A to be viewed, but the other part of light rays directly enters into the medium of the screen as stray light rays denoted by the reference character B, resulting the efficiency of light to be viewed at the viewing side of the screen being lowered.
In order to overcome the above-mentioned drawback, it has been proposed to make the apex angle .theta..sub.1 of the prisms small or to make the incidence angle .theta. of the light rays incident upon the screen.
However, it would be impractical to decrease the apex angle .theta..sub.1 since the manufacture of such a kind of screen is difficult if the apex angle .theta..sub.1 of the prisms becomes less than about 50 deg. Further, it is undesirable to increase the incidence angle .theta. since the above-mentioned uniform focussing problem would be serious if the incidence angle exceeds about 50 deg. at the center of the screen, resulting in difficulty in taking the above-mentioned correction measure at the light source P.